1. Field of the Invention
The present invention relates to tomography devices employing nuclear magnetic resonance for generating a tomographic image, and in particular to such devices wherein a projection of the nuclear spin is acquired on a frequency axis as a Fourier line and the image of the examined slice is calculated by Fourier transformation from the Fourier lines.
2. Description of the Prior Art
Nuclear magnetic resonance tomography devices operate based on the known principle that the hydrogen nuclear spin of an examination subject can be deflected from a preferred or equilibrium direction, imposed by a fundamental magnetic field, by means of a high-frequency excitation pulse. At the conclusion of the excitation pulse, the nuclear spins require a certain relaxation time to return to the equilibrium position. During this time, the spins precess with a frequency which is dependent on the strength of the fundamental magnetic field. This precessional motion can be detected with an RF antenna and associated receiver circuits. By superimposing a first field gradient on the uniform fundamental magnetic field, so that the magnetic field distribution spatially varies, identification of the spatial orientation of the spin (the gradient direction) can be undertaken by analyzing the measured frequency.
It is also known, before the signals in the first gradient are read out, to undertake a locus coding along a second spatial axis by briefly applying a second gradient field with variable amplitude or variable time duration. The second gradient field is applied orthogonally to the first gradient field, and slice images of the examination subject can be produced in this manner. Excitation of the nuclear spins in the slice of the examination subject is achieved because the fundamental magnetic field is influenced by a third gradient field disposed perpendicularly with respect to both the first and second gradient fields. The influence on the fundamental field is such that excitation of the spin occurs only in the slice of interest. This is possible because the excitation occurs with a frequency which is strictly allocated to the slice of interest. This method is described, for example, in German OS 26 11 497.
It is also known to store the acquired signals obtained during the spin relaxation time in a matrix in the form of Fourier lines, and to generate the image of the slice of interest by subjecting the lines to Fourier transformation.
Conventional devices operating as described above undertake a large number of sequential measuring events, so that a complete measurement for a particular layer is of a duration such that movement of the examination subject, for example, due to heart beat or respiration, takes place during the measurement and is present in the image as a disturbance or artifact. One method to avoid such disturbances is to control the acquisition of the measured values in dependence upon an EKG signal, or in dependence upon the patient's respiratory phase. When the acquisition of signals is controlled based on the respiratory phase, acquisition of measured values is only enabled for a relatively brief time span in each respiratory phase. This requires a considerable lengthening of the total measuring time, which is not always acceptable in practice.